The present invention pertains to a capacitive ignition system with ion-sensing comprising an ignition coil, with a primary winding that is connected to an energy source for providing the energy for a spark event and with a secondary winding having a first terminal connected to a spark plug so that a secondary voltage across the secondary winding is applied to the spark gap of the spark plug, an ionization current biasing and measurement circuitry on a secondary side of the ignition coil for providing a biasing voltage to the spark gap after the spark event for ion-sensing and a diode that is connected across the secondary winding. The invention pertains also to a method for damping AC ringing after occurrence of a spark event in a capacitive ignition system with ion-sensing.
It is well known that the combustion process of an internal combustion engine can be analysed using the ionization current across the spark gap of a spark plug. When the spark plug sparks the gas surrounding the spark gap is ionized. If a voltage is applied across the spark gap after the spark event has occurred, the ionized gas causes ionization current to flow across the spark gap that can be measured and analysed using suitable detection circuits. Measuring and analysing the ionization current (the so called ion-sensing) allows detecting misfire, engine knock, peak pressure, a deteriorating spark plug (plug fouling) and other characteristics of the engine or the combustion process. Information from ion-sensing enables also the correction or adjustment of ignition parameters in order to adapt to different load conditions or to improve the performance of the engine or to decrease emissions or fuel consumption, by influencing the air/fuel-ratio, for example. There are many known methods and systems in the prior art for detecting, measuring and analysing an ionization current.
An ignition system usually uses an ignition coil having a primary and secondary winding. The energy required for sparking is supplied from the primary winding to the secondary winding causing a secondary voltage across the secondary winding that is applied to the spark gap. Dependent on the energy source on the primary side for generating the primary voltage across the primary winding, it is differed between inductive ignition systems and capacitive ignition systems.
In an inductive ignition system the energy is stored in the primary winding which is released for sparking. To this end a primary switch in series with the primary winding is turned on for loading the coil primary that is connected to a supply voltage. The spark occurs when the primary switch is turned off. Inductive ignition, also with ion-sensing, is well known, e.g., from U.S. Pat. No. 5,230,240 A. In U.S. Pat. No. 5,230,240 A, a diode across the secondary winding is shown which prevents unwanted sparking when the primary switch turns on to load the coil primary. This diode is forward biased when the switch is turned on, and reverse biased when the switch is turned off. Hence, the diode conducts before the desired spark breakdown across the spark plug electrodes occurs. The diode across the secondary winding would need to conduct significant current every time the primary switch is turned on and would then need to dissipate the power again. This would significantly burden the diode, and a diode with high power rating would be required.
In a capacitive ignition system a storage capacitor on the primary side of the ignition coil stores the energy for sparking. The storage capacitor is discharged over the primary winding to generate the primary voltage across the primary winding, e.g., by turning on a switch that connects the capacitor with the primary winding. After the spark event, the capacitor is recharged for the next spark event. With capacitive ignition it is possible to generate short duration, high power sparks and, hence, is particularly suitable for igniting lean mixtures, such as in gas engines.
Capacitive ignition, also with ion-sensing, is well known, e.g., from WO 2013/045288 A1. In WO 2013/045288 A1 a resistor is connected in series with the spark plug for measuring the ionization current. The required bias voltage across the spark plug electrodes for ion-sensing is generated by repeatedly discharging the storage capacitor on the primary side after the initial spark breakdown.
A major challenge in combustion monitoring via ion-sensing of the spark gap is minimization of the associated ringing of the secondary voltage in the secondary winding of the ignition coil after the spark event. The coil secondary winding is an inductor with a DC current (direct current) flowing through it whenever the spark is created. When the spark goes out the secondary DC current drops to zero momentarily and as a result the charged inductance of the coil secondary winding tries to maintain the previous current flow. But because the secondary path is now highly resistant to the flow of DC current at the available secondary voltage, the only current which can flow is an AC current (alternating current) through the parasitic capacitance of the spark plug gap. This AC current causes the ringing of the secondary voltage. This parasitic AC current is often much larger in magnitude than the DC ion current which is the signal of interest with ion-sensing, which makes ion-sensing difficult. This phenomenon has traditionally been managed by a number of different approaches, namely reduced coil impedance and active “turn-off” circuits on the primary side of the circuit. Reduced coil impedance can significantly impact ignition performance as the coil with reduced coil impedance typically delivers very short duration sparks with limited output energy. Active “turn-off” circuits on the primary side, on the other hand, can improve the ringing behaviour on the secondary winding, but are cumber-some to implement effectively and have limited benefit.
From EP 1 990 813 A1 an inductive ignition system with ion-sensing and an apparatus for reducing ringing of the secondary voltage is known. For ion-sensing a capacitor on the secondary side of the ignition coil is charged during the flow of a spark current. After the spark breakdown occurred, the capacitor is discharged to generate the bias voltage across the spark plug electrodes for detecting the ionization current that is measured. For reducing the ringing of the secondary voltage, that would influence the measurement of the ionization current, an additional control winding in series with a diode are arranged on the primary side of the ignition coil. This diode is oriented so that it is forward biased only when a current opposite to the spark current, e.g., an ionization current, flows and, hence, does not conduct during the spark event. After the spark goes out, the control winding and the diode cooperate to dissipate residual electrical charge in the coil in order to limit the ringing. However, the diode introduces an incremental parasitic loss during charging of the ignition coil primary that will detrimental-ly increase the amount energy required for charging the coil primary.
Another capacitive ignition system with ion-sensing is shown in EP 879 355 B1, which uses an additional energy source on the secondary side for generating a high current spark arc and also for generating the required bias voltage across the spark plug electrodes for ion-sensing. The energy source of the primary side is used solely for creating a spark across the spark gap. To this end a high-voltage diode is connected across the secondary winding. If the capacitor on the primary side is discharged for sparking, a high voltage is created on the secondary winding. This high voltage is also applied across the spark gap and ionizes the matter surrounding the spark gap and creates the spark. Once the spark gap is ionized, the secondary side energy source connected to the coil secondary provides the required current, which flows over the ionized spark gap, to generate the arc for the spark event. This spark current flows also over the forward-biased high-voltage diode, which ensures that the secondary side energy source is decoupled from the primary side of the ignition coil. The high-voltage diode is used to supply the power to the spark. The energy for creating the spark which is supplied by the secondary side energy source connected to the coil secondary is quickly dissipated in the secondary winding and the high-voltage diode. In addition, after the spark event, the secondary side energy source provides also the ionization current for ion sensing. This ionization current flows again over the forward-biased high-voltage diode and, during ion-sensing, the high-voltage secondary side is again decoupled from the primary side of the ignition coil to pre-vent undesired cross conduction or interaction of the two separate isolated energy sources. The additional energy source increases the complexity of the ignition system with regard to hardware, as well as with regard to timing and control of the energy sources. The secondary winding and the high-voltage diode are significantly thermally burdened. Therefore, both the ignition coil and the high-voltage diode must be designed or chosen to withstand this high thermal load caused by the fact that the secondary side high-voltage diode conducts both the spark current and the ionization current. In EP 879 355 B1 a low pass filter is used to condition the ionization current signal. Because of the polarity of the secondary side energy source, the secondary ringing voltages are not suppressed by the high-voltage diode which can be seen in the waveforms of FIGS. 5a and 5b of EP 879 355 B1.
It is an object of the present invention to provide a method and an apparatus for easily reducing AC ringing of the secondary voltage after the spark event in a capacitive ignition system.